High Temperature Thermoelectrics

ABSTRACT

In accordance with one embodiment of the present disclosure, a thermoelectric device includes a plurality of thermoelectric elements that each include a diffusion barrier. The diffusion barrier includes a refractory metal. The thermoelectric device also includes a plurality of conductors coupled to the plurality of thermoelectric elements. The plurality of conductors include aluminum. In addition, the thermoelectric device includes at least one plate coupled to the plurality of thermoelectric elements using a braze. The braze includes aluminum.

GOVERNMENT RIGHTS

A portion or all of this disclosure may have been made with Governmentsupport under government contract number TCS-236-36 awarded by theUnited States Department of Energy, and under government contract numberW909MY09C0061 awarded by the United States Army of the United StatesDepartment of Defense. The Government may have certain rights in thisdisclosure.

TECHNICAL FIELD

This disclosure relates in general to thermoelectric devices, and moreparticularly to high temperature thermoelectrics.

BACKGROUND OF THE DISCLOSURE

The basic theory and operation of thermoelectric devices has beendeveloped for many years. Presently available thermoelectric devicesused for cooling typically include an array of thermocouples whichoperate in accordance with the Peltier effect. Thermoelectric devicesmay also be used for heating, power generation and temperature sensing.

Thermoelectric devices may be described as essentially small heat pumpswhich follow the laws of thermodynamics in the same manner as mechanicalheat pumps, refrigerators, or any other apparatus used to transfer heatenergy. A principal difference is that thermoelectric devices functionwith solid state electrical components (thermoelectric elements orthermocouples) as compared to more traditional mechanical/fluid heatingand cooling components.

Thermoelectric materials such as alloys of Bi₂Te₃, PbTe and BiSb weredeveloped thirty to forty years ago. More recently, semiconductor alloyssuch as SiGe have been used in the fabrication of thermoelectricdevices. Typically, a thermoelectric device incorporates both a P-typesemiconductor and an N-type semiconductor alloy as the thermoelectricmaterials.

As cooling applications progressively require thermoelectric devices tooperate at higher temperatures, existing techniques have been unable toproduce effective solutions.

SUMMARY OF THE DISCLOSURE

In some embodiments, certain disadvantages and problems associated withusing thermoelectric devices in high temperature environments have beensubstantially reduced or eliminated.

In accordance with one embodiment of the present disclosure, athermoelectric device includes at least one plate and a plurality ofconductors formed on the at least one plate. The plurality of conductorsincludes aluminum. The thermoelectric device includes a plurality ofthermoelectric elements that each include a diffusion barrier coupled tothe plurality of conductors using a braze. The diffusion barrierincludes a refractory metal. The braze includes aluminum.

In some embodiments, the refractory metal may include molybdenum. The atleast one plate may include aluminum oxide or aluminum nitride. Thebraze may include aluminum silicon. The thermoelectric device may alsoinclude a lead. The lead may be resistance welded to the at least oneplate.

In accordance with another embodiment of the present disclosure, amethod of forming a thermoelectric generator includes applying adiffusion barrier to a plurality of thermoelectric elements. Thediffusion barrier includes a refractory metal. The method also includesforming a plurality of conductors on at least one plate. The pluralityof conductors include aluminum. In addition, the method includescoupling the plurality of conductors to the plurality of thermoelectricelements using a braze. The braze includes aluminum.

Technical advantages of certain embodiments of the present disclosureinclude enabling extended temperature operation superior to existingtechniques. Some existing thermoelectric devices experience rapiddegradation due to thermal stresses. Certain embodiments of the presentdisclosure provide for the accommodation of thermal expansion duringoperation.

Other technical advantages of the present disclosure will be readilyapparent to one skilled in the art from the following figures,descriptions, and claims. Moreover, while specific advantages have beenenumerated above, various embodiments may include all, some, or none ofthe enumerated advantages.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present disclosure and itsadvantages, reference is now made to the following description, taken inconjunction with the accompanying drawings, in which:

FIG. 1 illustrates one embodiment of a thermoelectric device including aplurality of thermoelectric elements disposed between a cold plate and ahot plate;

FIG. 2 illustrates one embodiment of a thermoelectric device capable ofoperating in high temperatures; and

FIG. 3 is a flowchart illustrating one embodiment of forming athermoelectric device.

DETAILED DESCRIPTION OF THE DISCLOSURE

FIG. 1 illustrates one embodiment of a thermoelectric device 20including a plurality of thermoelectric elements 22 disposed between acold plate 24 and a hot plate 26. Electrical connections 28 and 30 areprovided to allow thermoelectric device 20 to be electrically coupledwith an appropriate source of DC electrical power or to allowthermoelectric device 20 to be electrically coupled to one or moredevices that use, transform, or store power when thermoelectric device20 operates as a generator.

Thermoelectric device 20 may be used as a heater, cooler, electricalpower generator, and/or temperature sensor. If thermoelectric device 20were designed to function as an electrical power generator, electricalconnections 28 and 30 would represent the output terminals from such apower generator operating between hot and cold temperature sources.

FIG. 2 illustrates one embodiment of thermoelectric device 200 capableof operating in high temperatures. This may be an example of howthermoelectric device 20 may be implemented. Thermoelectric device 200may include thermoelectric elements 202 fabricated from dissimilarsemiconductor materials such as N-type thermoelectric elements 202 a andP-type thermoelectric elements 202 b. Thermoelectric elements 202 aretypically configured in a generally alternating N-type element to P-typeelement arrangement and typically include an air gap 204 disposedbetween adjacent N-type and P-type elements. In many thermoelectricdevices, thermoelectric materials with dissimilar characteristics areconnected electrically in series and thermally in parallel.

Examples of thermoelectric devices and methods of fabrication are shownin U.S. Pat. No. 5,064,476 titled Thermoelectric Cooler and FabricationMethod; U.S. Pat. No. 5,171,372 titled Thermoelectric Cooler andFabrication Method; and U.S. Pat. No. 5,576,512 entitled ThermoelectricApparatus for Use With Multiple Power Sources and Method of Operation.

N-type semiconductor materials generally have more electrons thannecessary to complete the associated crystal lattice structure. P-typesemiconductor materials generally have fewer electrons than necessary tocomplete the associated crystal lattice structure. The “missingelectrons” are sometimes referred to as “holes.” The extra electrons andextra holes are sometimes referred to as “carriers.” The extra electronsin N-type semiconductor materials and the extra holes in P-typesemiconductor materials are the agents or carriers which transport ormove heat energy between cold side or cold plate 206 and hot side or hotplate 208 through thermoelectric elements 200 when subject to a DCvoltage potential. These same agents or carriers may generate electricalpower when an appropriate temperature difference is present between coldside 206 and hot side 208. Leads 214 may be coupled to plate 208 in amanner that withstands high temperature environments, such as resistancewelding, tungsten inert gas (TIG) welding, and laser welding.

In some embodiments, thermoelectric elements 202 may include hightemperature thermoelectric material. Examples of high temperaturethermoelectric materials include lead telluride (PbTe), lead germaniumtelluride (PbGeTe), TAGS alloys (such as (GeTe)_(0.85)(AgSbTe2)_(0.15)),bismuth telluride (Bi₂Te₃), and skutterudites.

In some embodiments, thermoelectric elements 202 may include a diffusionbarrier that includes refractory metals (e.g., a metal with a meltingpoint above 1,850° C.). Suitable refractory metals may include thosethat are metallurgically compatible with high temperature thermoelectricmaterials and metallurgically compatible with other components ofthermoelectric device 200. For example, a molybdenum diffusion barriermay be used. This may be advantageous in that molybdenum may bemetallurgically compatible with various aspects of thermoelectric device200. For example, as further discussed below, thermoelectric device 200may include an aluminum braze that is metallurgically compatible with amolybdenum diffusion barrier. Such a diffusion barrier may prevent orreduce the chance or occurrence of Kirkendall voiding in thermoelectricdevice 200. Other suitable examples of a diffusion barrier that hassimilar properties to molybdenum include tungsten and titanium.

In some embodiments, alternating thermoelectric elements 202 of N-typeand P-type semiconductor materials may have their ends connected byelectrical conductors 210. Conductors 210 may be metallizations formedon thermoelectric elements 202 and/or on the interior surfaces of plates206 and 208. Conductors 210 may include aluminum. Ceramic materials maybe included in plates 206 and 208 which define in part the cold side andhot side, respectively, of thermoelectric device 200. In someembodiments, the ceramic materials may provide electrical isolation fromhot and cold side sources. Aluminum metallized ceramics may accommodatethermal stresses (i.e., due to high temperature exposure) of theceramic/aluminum bond. Examples of suitable ceramic materials includealuminum oxide, aluminum nitride, and beryllium oxide.

In some embodiments, thermoelectric elements 202 may be coupled toplates 206 and 208 using medium 212. Medium 212 may include brazesand/or solders. For example, aluminum-based brazes and/or solders may beused, such as aluminum silicon (AlSi) braze family and/or zinc-aluminum(ZnAl) solder. In some embodiments, using such brazes and/or solders mayprovide for high temperature operation and allow for flexible joints.Kirkendall voiding may be prevented or reduced.

In some embodiments, using one or more of the configurations discussedabove, thermoelectric device 200 may be suitable as a fixed-joint, hightemperature thermoelectric generator that is capable of being used inhigh temperature applications. For example, a thermoelectric generatorbuilt using skutterudite thermoelectric elements that include amolybdenum diffusion barrier, conductors formed by aluminummetallizations, and aluminum based brazes may result in a device thatcan operate with at least one of its plates (such as plates 206 or 208)at a temperature greater than 500 degrees Celsius. As another example, athermoelectric generator built using bismuth telluride thermoelectricelements that include a molybdenum diffusion barrier, conductors formedby aluminum metallization, and zinc-aluminum (ZnAl) solder may result ina device that can operate with at least one of its plates (such asplates 206 or 208) at a temperature greater than 300 degrees Celsius.

FIG. 3 is a flowchart illustrating one embodiment of forming anthermoelectric device. For example, the steps illustrated in FIG. 3 maybe used to form a thermoelectric generator. In general, the stepsillustrated in FIG. 3 may be combined, modified, or deleted whereappropriate, and additional steps may also be added to the exampleoperation. Furthermore, the described steps may be performed in anysuitable order.

At step 310, in some embodiments, a diffusion barrier may be applied toone or more thermoelectric elements. The diffusion barrier may be orinclude refractory metals, such as molybdenum, tungsten, and titanium.For example, a molybdenum diffusion barrier metallization may be appliedat this step.

At step 320, in some embodiments, conductors may be formed. Theconductors may be metallizations formed on the thermoelectric elementsand/or formed on plates (e.g., on the interior surfaces of the plates).The plates may be ceramic plates. The conductors may be formed ofaluminum.

At step 330, in some embodiments, plates may be coupled to thethermoelectric elements. For example, the thermoelectric elements may becoupled to the interior surfaces of two plates where conductors havebeen formed (e.g., at step 320) such that the thermoelectric elementsmay be disposed between the two plates. The thermoelectric elements maybe coupled to conductors on the plates such that an N-typethermoelectric element is coupled to a P-type thermoelectric element.The plates may be coupled to the thermoelectric elements using brazesand/or solders. For example, aluminum-based brazes and/or solders may beused, such as aluminum silicon (AlSi) braze family and/or zinc-aluminum(ZnAl) solder.

At step 340, in some embodiments, leads may be coupled to at least oneof the plates. This may be performed using resistance welding, tungsteninert gas (TIG) welding, or laser welding. The leads may be coupled suchthat electricity generated by the thermoelectric device may be sentthrough the leads to another device. As another example, the leads maybe coupled such that electricity may be applied to the thermoelectricdevice.

Although the present disclosure has been described with severalembodiments, a myriad of changes, variations, alterations,transformations, and modifications may be suggested to one skilled inthe art, and it is intended that the present disclosure encompass suchchanges, variations, alterations, transformations, and modifications asfall within the scope of the appended claims.

1-20. (canceled)
 21. A thermoelectric device comprising: a first set ofplates comprising a first plurality of conductors directly bonded to thefirst set of plates, the first plurality of conductors consistingessentially of aluminum, the first set of plates being electricallyinsulative; a second set of plates comprising a second plurality ofconductors directly bonded to the second set of plates, the secondplurality of conductors consisting essentially of aluminum, the firstset of plates being electrically insulative; a plurality ofthermoelectric elements situated between the first set of plates and thesecond set of plates, the plurality of thermoelectric elements coupledto the first plurality of conductors and the second plurality ofconductors; and a plurality of brazes comprising aluminum, the pluralityof brazes situated at interfaces between the plurality of thermoelectricelements and the first plurality of conductors.
 22. The thermoelectricdevice of claim 21 wherein the plurality of thermoelectric elementscomprises a diffusion barrier.
 23. The thermoelectric device of claim 22wherein the diffusion barrier comprises a refractory metal.
 24. Thethermoelectric device of claim 22 wherein the diffusion barriercomprises titanium.
 25. The thermoelectric device of claim 21 whereinthe first set of plates plate and the second set of plates comprise amaterial selected from the group consisting of: aluminum oxide andaluminum nitride.
 26. The thermoelectric device of claim 21 wherein theplurality of brazes comprise aluminum silicon.
 27. The thermoelectricdevice of claim 21 further comprising solder, the solder situated atinterfaces between the plurality of thermoelectric elements and thesecond plurality of conductors; and wherein the second set of platesconsists of one plate.
 28. A method of forming a thermoelectric device,comprising: directly bonding a first plurality of conductors on a firstset of plates, the first plurality of conductors consisting essentiallyof aluminum, the first set of plates being electrically insulative;directly bonding a second plurality of conductors on a second set ofplates, the second plurality of conductors consisting essentially ofaluminum, the first set of plates being electrically insulative;applying a plurality of brazes on the first plurality of conductors, theplurality of brazes comprising aluminum; situating a plurality ofthermoelectric elements between the first set of plates and the secondset of plates, the plurality of brazes situated at interfaces betweenthe plurality of thermoelectric elements and the first plurality ofconductors; and coupling the first plurality of conductors and thesecond plurality of conductors to the plurality of thermoelectricelements.
 29. The method of claim 28 further comprising applying adiffusion barrier to the plurality of thermoelectric elements.
 30. Themethod of claim 29 wherein the diffusion barrier comprises a refractorymetal.
 31. The method of claim 29 wherein the diffusion barriercomprises titanium.
 32. The method of claim 28 wherein the first set ofplates and the second set of plates comprise a material selected fromthe group consisting of: aluminum oxide and aluminum nitride.
 33. Themethod of claim 28 wherein the plurality of brazes comprise aluminumsilicon.
 34. The method of claim 28 further comprising adding solder tointerfaces between the plurality of thermoelectric elements and thesecond plurality of conductors; and wherein the second set of platesconsists of one plate.
 35. A thermoelectric generator comprising: afirst set of plates comprising a first plurality of conductors directlybonded to the first set of plates, the first plurality of conductorsconsisting essentially of aluminum, the first set of plates beingelectrically insulative; a second set of plates comprising a secondplurality of conductors directly bonded to the second set of plates, thesecond plurality of conductors consisting essentially of aluminum, thefirst set of elates being electrically insulative; a plurality of P-typeand N-type thermoelectric elements situated between the first set ofplates and the second set of plates, the plurality of thermoelectricelements coupled to the first plurality of conductors and the secondplurality of conductors; a plurality of brazes comprising aluminum, theplurality of brazes situated at interfaces between the plurality ofthermoelectric elements and the first plurality of conductors; andwherein the thermoelectric generator is configured to operate while thefirst set of plates is at a temperature above 300 degrees Celsius. 36.The thermoelectric generator of claim 35 wherein the plurality ofthermoelectric elements comprises a diffusion barrier.
 37. Thethermoelectric generator of claim 36 wherein the diffusion barriercomprises a refractory metal.
 38. The thermoelectric generator of claim36 wherein the diffusion barrier comprises titanium.
 39. Thethermoelectric generator of claim 35 wherein the first set of plates andthe second set of plates comprise a material selected from the groupconsisting of: aluminum oxide and aluminum nitride.
 40. Thethermoelectric generator of claim 35 wherein the plurality of brazescomprises aluminum silicon.
 41. The thermoelectric generator of claim35, wherein the thermoelectric generator is configured to operate whilethe first set of plates is at a temperature above 500 degrees Celsiusand wherein the second set of plates consists of one plate.